Multi-section attenuating networks



June 2 1967 J D- MIDDLETON MULTI-SECTION ATTENUATING NETWORKS Filed March 1, 1965 F PRIOR ART 02 -L. cr T W Amswev:

Iuvawroe jg: Jwzd/ 76% I United States Patent 3,328,678 MULTI-SECTION ATTENUATIN G NETWORKS John David Middleton, Cuilley, Hertfordshire, England, assignor to Marconi Instruments Limited, London, England, a British company Filed Mar. 1, 1965, Ser. No. 435,846 Claims priority, application Great Britain, June 3, 1964, 22,985/ 64 4 Claims. (Cl. 323-74) ABSTRACT OF THE DISCLOSURE A wide frequency band attenuator comprising two or more sections each of which has a series resistance in parallel with a condenser and two shunt condensers one one each side of the resistance. An electrostatic screen is preferably provided between each two successive sections and a resistance is provided across the output of the attenuator. The output resistance, the series resistances and the condensers are of values interrelated to provide a cascade capacitance-graded resistance network.

This invention relates to attenuating networks and more particularly to high impedance attenuating networks of relatively high attenuation ratio suitable for use in wave form displaying and similar measuring oscill-oscopes. Such Oscilloscopes may be required to handle any frequency within a very wide range extending from quite low frequencies up to high frequency transients and' attenuators for use in such oscilloscopes must therefore be able to provide attenuation over a correspondingly wide range with a minimum of distortion and a minimum of frequency-dependent response or frequency sensitivity. The present invention seeks to satisfy this difiicult requirement to a high degree by means of relatively simple and inexpensive networks of high attenuation ratio and which, for a given requirement as to ratio, shall. involve the use of circuit elements of only relatively small tolerances as compared with those of elements which would 'be employed in comparable known networks.

The invention is illustrated in and explained in connection with the accompanying diagrammatic drawings in which FIG. 1 shows a typical known attenuator section; FIG. 2 shows an attenuator in accordance with this invention and having two sections; FIG. 3 shows a multisectioned attenuator in accordance with this invention but with the electrostatic screens omitted from the figure; and FIG. 4 indicates graphically practical results obtainable by use of the invention.

FIG. 1 shows a typical known attenuator section as employed in current practice in attenuators for measuring oscilloscopes. In this section a series resistance R is connected in the live wire between one of the input terminals 1 and one of the output terminals 2; and adjustable condenser C is connected across R; and shunt condensers are connected respectively across the input and output ends of the section, a further resistance being connected across the out-put of the section. There may be a number of sections as so far described in cascade. As, however, the number of sections is increased, the effect produced by the tolerances of the resistances in the sections on the attenuation ratio also increases and, with a large number of sections and a requirement of high attenuation ratio, the network becomes very expensive if not impractical.

To take a practice example for oscilloscope work suppose that a single section attenuator as shown in FIG. 1 is required to give an attenuation of 1000' times. This 3,328,678 Patented June 27, 1967 would involve having a ratio of value of shunt capacity at the output end to series capacity of about 1000/ 1. In practice, it is virtually impossible to have a series capacity of less thanabout 1 pf. for the unavoidable stray capacitance across the series resistance has to be taken into account. The leads by which the output shunt condenser is connected of course present inductance and if one assumes a lead length of A" a condenser of 1000 pf. will resonate, with its own leads, at about 50 mc./s. This consideration alone sets the upper frequency limit of usefulness of an attenuator so designed to about 10 mc./s.

According to this invention an attenuating network comprises at least two sections in cascade each including a series, resistance, a condenser in parallel with said resistance and two shunt condensers one on each side of said resistance and one of which may be common with a shunt'condenser of the next section; and a shunt resistance across the output end of the last of the sections, the elements in the network being dimensioned so as substantially to satisfy the following equations namely:

-1) RPRTW and Rt= N where C is the value of each shunt condenser other than the one nearest the input end and the one nearest the output end of the network; C is the value of each condenser in parallel with a series resistance; C is the value of the shunt condenser nearest the output end of the network; R is the value of the r" series resistance counting from the input end of the network; N isvthe required total attenuation (in times) of the network and is equal to n where n is the attenuation of each section (including any effective loading); R, is the value of the shunt resistance across the output endof the whole network; and R is the total input resistance of the network.

Preferably an electrostatic screen is provided between each two successive sections.

In normal practice the condensers across the series resistances will be adjustable and arranged for factory or laboratory adjustment.

FIG. 2 shows a two section network in accordance With the invention. As will be seen there is a shunt condenser C across the input terminals and which is dimensioned to provide a desired input capacitance for the network; two series resistances R and R with an earthed electrostatic screen S between them; a condenser C in parallel with each series resistance; and shunt condensers C and C, connected respectively to the juuction point of the two series resistances and the output side thereof. Condenser C is a shunt condenser common to both sections. A resistance R, is connected across the output end of the network. The screen S need not be earthed. It could be connected, instead to the other side of condenser C with very little diiference to the performance obtained though, in general, the earthed arrangement is preferred. The elements in the network (other than C are dimensioned to satisfy the foregoing equations. Since there are only two sections in the embodiment of FIG. 2, n= /N.

FIG. 3 shows a multi-sectioned network. So as not to complicate FIG. 3, the electrostatic screens, which are provided between the series resistances are not shown in the figure. The first three series resistances are referenced R R R and the last three are not referenced. The elements (other than C are dimensioned to satisfy the equations given above.

It will now 'be shown, by the aid of a non-limiting example, that the values of capacity and resistance required to carry out this invention are quite practical and convenient. Assume that the total attenuation N is required is 1000 times; that the number of sections is to be 2; and that the total input resistance R required is to be 1 m9. Then n= /N- 31.6.

Let C; be chosen at 1.5 pf.

FIG. 4 shows graphically and by way of practical ex ample, the general measure of improvement obtainable by the invention. In this figure the full line curve shows an input pulse of amplitude 80 volts and 100 nano-secs. wide provided from a mercury switch. The dotted curve shows the pulse, after attenuation by a good quality attenuator at present in commercial use, amplified by a high quality amplifier designed to handle frequencies up to 30 mc./s. and displayed by a cathode ray tube. The broken line curve shows the same pulse, after attenuation by an attenuator in accordance with this invention, amplified by the same amplifier and displayed by the same tube. It will be seen that the great advantage the invention possesses is that almost any number of sections may be employed, so that condensers C and C may be kept down to desirably small sizes while at the same time, however many sections are employed, a desired accuracy of attenuation ratio is attainable with resistance of a relatively low tolerance for the maximum inaccuracy of attenuation ratio is only twice the tolerance of the individual resistances.

I claim:

1. An attenuating network comprising at least two sections in cascade each including a series resistance, a condenser in parallel with said resistance and two shunt condensers one on each side of said resistance, and a shunt resistance across the output end of the last of the sections, the elements in the network being dimensioned so as substantially to satisfy the following equations namely:

where C is the value of each shunt condenser other than the one nearest the input end and the one nearest the output end of the network; C is the value of each condenser in parallel with a series resistance; C is the value of the shunt condenser nearest the output end of the network; R is the value of the r series resistance counting from the input end of the network; N is the required total attenuation, in times, of the network and is equal to n where n is the attenuation of each section including efiective loading; R is the value of the shunt resistance across the output end of the whole network; and R is the total input resistance of the network.

2. A network as claimed in claim 1 wherein an electrostatic screen is provided between each two successive sections.

3. A network as claimed in claim 2 wherein the condensers across the series resistances are adjustable.

4. A network as claimed in claim 1 wherein shunt condensers in adjacent sections are common.

References Cited UNITED STATES PATENTS 3,267,355 8/1966 Dl'anetz 393-94 X FOREIGN PATENTS 570,966 7/ 1945 Great Britain.

JOHN F. COUCH, Primary Examiner.

A. D. PELLIN-EN, Assistant Examiner. 

1. AN ATTENUATING NETWORK COMPRISING AT LEAST TWO SECTIONS IN CASCADE EACH INCLUDING A SERIES RESISTANCE, A CONDENSER IN PARALLEL WITH SAID RESISTANCE AND TWO SHUNT CONDENSERS ONE ON EACH SIDE OF SAID RESISTANCE, AND A SHUNT RESISTANCE ACROSS THE OUTPUT END OF THE LAST OF THE SECTIONS, THE ELEMENTS IN THE NETWORK BEING DIMENSIONED SO AS SUBSTANTIALLY TO SATISFY THE FOLLOWING EQUATIONS NAMELY: 